1. Field of Invention
The invention relates to removal of gas contaminants, and more particularly, to removal of gas contaminants by sparging the contaminated gas through a fluid filled container having biomass suspended therein, where the gas contaminants have a low Henry's coefficient.
2. Prior Art
Removal of gas contaminants generated by industrial processes, treatment of contaminated soils, wastewater treatment plants or other sources of gas contaminants has been made desirable with the increased awareness for environmental integrity, and in some cases, made mandatory by law, particularly more stringent clean air requirements. Various methods have been developed for treatment of contaminated gas streams, such as filtering through activated carbon (adsorption processes), absorption methods, catalytic destruction, thermal destruction (incineration), vapor phase oxidation methods, and biological metabolic or cometabolic methods.
Biological metabolic/cometabolic methods include packed bed biofilters, bioscrubbers, and biotrickling filters. Both biofilters and biotrickling filters are not submerged reactors and contain a significant volume void of liquids or solids. Biofilters treat contaminated gases by moving the gas stream through generally stationary biomass where biological metabolic processes convert the contaminants into less toxic or harmful substances (such as compost filtering). Bioscrubbers generally have biomass suspended in a mobile liquid phase (the solution), the solution is then sprayed into the moving gas stream. Liquid droplets of solution contact the gas contaminants, providing a surface area across which transfer of the gas contaminants into the liquid phase occurs. Bioscrubbers require circulation of both the gas stream and the solution. Biotrickling filters contain inert packing material to which the biomass attaches. Liquid, containing nutrients and buffers, is circulated through the packing material, and contaminated gases are mixed with the circulating fluid. The biomass is considered stationary. Flowing contaminated gases through fluids, primarily through wastewater treatment plant treatment aeration basins containing activated sludge, has also been tried using small gas diffuser. While found to be effective for certain compounds, the depths of these basins, generally 15 feet, make stand alone bioreactors expensive. Additionally, with insufficient oxygen diffusion into the basin near the gas contaminant, such a process is inefficient. Finally, treatment basins have a variety of available food sources for microorganisms, making it difficult to utilize a treatment basin bioreactor to efficiently remove compounds which are more difficult to metabolize than other compounds present in wastewater. The wide variety of food sources available in such a system will generally result in a biomass that will metabolize certain contaminants preferentially over other contaminants. This variance in metabolic rates of differing compounds can result in inefficient or insufficient metabolism of the gas contaminant.
All of these methods present difficulties. Biofilters can have an excessively large "footprint", the media decays and must be refreshed, and it is difficult to provide nutrient supplements to the growing stationary biomass. Bioscrubbers, require two pumps and generally has higher operating and maintenance cost than biofilter, due in part to the increased mechanical complexity of the system. Further, bioscrubbers provide low surface area for mass transfer of the contaminants. Biotrickling filters and biofilters can have media plugging problems due to excess biogrowth and gas short circulating problems, creating uncertainty in the performance reliability of these devices.